Refrigerant vapor compression systems are well known in the art and commonly used for conditioning air (or other secondary media) to be supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Refrigerant vapor compression systems are also commonly used in transport refrigeration systems for refrigerating air supplied to a temperature controlled cargo space of a truck, trailer, container or the like for transporting perishable items and in commercial refrigeration systems for cooling air supplied to a temperature controlled space in a cold room, a beverage cooler, a diary case or a refrigerated merchandiser for displaying perishable foods item in a chilled or frozen state, as appropriate. Typically, these refrigerant vapor compression systems include a compressor, a condenser, an evaporator, and an expansion device. Commonly, the expansion device, typically a fixed orifice, a capillary tube, a thermostatic expansion valve (TXV) or an electronic expansion valve (EXV), is disposed in the refrigerant line upstream, with respect to refrigerant flow, of the evaporator and downstream of the condenser. These basic refrigerant system components are interconnected by refrigerant lines in a closed refrigerant circuit, arranged in accord with known refrigerant vapor compression cycles.
Traditionally, most of these refrigerant vapor compression systems operate at subcritical refrigerant pressures. Refrigerant vapor compression systems operating in the subcritical range are commonly charged with fluorocarbon refrigerants such as, but not limited to, hydrochlorofluorocarbons (HCFCs), such as R22, and more commonly hydrofluorocarbons (HFCs), such as R134a, R410A and R407C. Although HFC refrigerants are more environmentally friendly than the chlorine containing HCFC refrigerants that they replaced, “natural” refrigerants, such as carbon dioxide (also referred to as R744), are being turned to for use in air conditioning and transport refrigeration systems instead of HFC refrigerants.
Because carbon dioxide has a low critical point, most refrigerant vapor compression systems charged with carbon dioxide as the refrigerant are designed for operation in the transcritical pressure regime. In refrigerant vapor compression systems operating in a transcritical cycle, the refrigerant discharged from a compressor is a vapor having a temperature and pressure in excess of the refrigerant's critical point. As in conventional refrigerant vapor compression systems operating in a subcritical cycle, refrigerant vapor compression systems operating in a transcritical cycle, include a compression device, a heat rejecting heat exchanger functioning as a gas cooler rather than a condenser, an evaporator, and an expansion device arranged in accord with known refrigerant vapor compression cycles. Typically, the expansion device is a thermostatic expansion valve (TXV) or an electronic expansion valve (EXV) disposed in the refrigerant line upstream, with respect to refrigerant flow, of the evaporator and downstream of the gas cooler.
Refrigerant vapor compression systems utilizing a low critical point refrigerant, such as carbon dioxide, often employ a two-stage compression system, either a pair of compressors disposed in series flow arrangement with respect to refrigerant flow or a single compressor having at least two compression stages. To improve the refrigerant system performance and to control the temperature of the refrigerant vapor discharged from the final stage of the compression system over a wide range of operating conditions, commonly referred to as the discharge pressure or the high-side pressure, it is known to equip such systems with an economizer cycle incorporating a refrigerant-to-refrigerant economizer heat exchanger. The economizer heat exchanger is generally disposed in the refrigerant circuit intermediate the gas cooler and the evaporator to further cool the refrigerant in the main circuit exiting the gas cooler, and to return an expanded (to an intermediate pressure) portion of refrigerant having traversed the economizer heat exchanger in heat transfer interaction with the refrigerant in the main circuit as the supplementary cooling fluid to the compressor. Typically, the refrigerant vapor returned to the compressor is injected into an intermediate stage in the compression process, either through an injection port or ports opening into an intermediate pressure stage of the compression chamber (or chambers) of a single compressor or, in the case of a multiple compressor system, into a refrigerant line extending between the discharge outlet of the upstream compressor and the suction inlet of the downstream compressor. Additionally, liquid refrigerant may be taken from a location downstream of the heat rejecting heat exchanger and returned to the compressor, generally through a separate injection port or ports opening to an intermediate stage of the compression process. It is to be understood that the vapor injection in the economizer cycle and the liquid injection can potentially take place at different intermediate pressures in the compression process, especially in the case when vapor and liquid are injected through separate lines.
For example, U.S. Pat. No. 6,571,576 discloses a refrigerant vapor compression system operating in a subcritical cycle and equipped with an economizer heat exchanger wherein vapor refrigerant and liquid refrigerant are returned to an intermediate stage of the compression process through one or more economizer injection ports provided in the compressor. To provide the refrigerant vapor for injection into the compressor, liquid refrigerant is taken from the refrigerant circuit at a location downstream of the condenser, expanded to an intermediate pressure and lower temperature by means of an expansion valve to form a refrigerant liquid/vapor mixture which is thereafter passed through the economizer heat exchanger in heat exchange relationship with the main flow of refrigerant liquid. In traversing the economizer heat exchanger, this refrigerant liquid/vapor mixture extracts heat from the main flow of refrigerant liquid, further cooling this liquid, thereby evaporating any remaining liquid component in the two-phase mixture and typically further heating the vapor. The refrigerant vapor leaving the economizer heat exchanger is then injected into the compressor through the economizer injection ports at the intermediate (between suction and discharge) pressure. Additionally, liquid refrigerant is selectively taken from the refrigerant circuit at a location downstream of the condenser and mixed into the refrigerant vapor being passed from the economizer to the compressor and injected into an intermediate pressure stage of the compression process together with the refrigerant vapor through the same economizer injection ports.
U.S. Patent Application Publication No. US 2005/0044885 A1 discloses a transcritical cycle for a carbon dioxide refrigerant vapor compression system including a compressor, a gas cooler, a flash tank economizer, an evaporator, a first expansion valve upstream of the flash tank economizer and a second expansion valve downstream of the flash tank economizer. Refrigerant passing from the gas cooler to the evaporator is expanded to a lower pressure by the first expansion valve before entering the flash tank economizer wherein the refrigerant separates into a liquid component and a vapor component. The liquid refrigerant passes from the flash tank economizer through and is further expanded in the second expansion valve before traversing the evaporator. The vapor refrigerant returns to the compressor at some intermediate pressure.
U.S. Pat. No. 6,880,357 discloses a refrigerant cycle apparatus, using carbon dioxide as the refrigerant, that is equipped with an expander and optionally a sub-expander disposed in the refrigerant circuit between an outdoor heat exchanger and an indoor heat exchanger. High pressure refrigerant is taken from the refrigerant circuit and injected into an intermediate pressure stage of the expander. Power recovered during the expansion process in the expander and sub-expander may be used to drive the compressor or an electricity generator.